Lattice Strain Induced Remarkable Enhancement in PiezoelectricPerformance of ZnO-Based Flexible NanogeneratorsYang Zhang,† Caihong Liu,† Jingbin Liu,§ Jie Xiong,§ Jingyu Liu,† Ke Zhang,† Yudong Liu,†

Mingzeng Peng,† Aifang Yu,† Aihua Zhang,† Yan Zhang,† Zhiwei Wang,† Junyi Zhai,*,†

and Zhong Lin Wang*,†,‡

†Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China‡School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States§State Key Laboratory of Electronic Thin Films and Integrated Device, University of Electronic Science and Technology of China,Chengdu 610054, China

*S Supporting Information

ABSTRACT: In this work, by employing halogen elements (fluorine,chlorine, bromine, and iodine) as dopant we demonstrate a unique strategyto enhance the output performance of ZnO-based flexible piezoelectricnanogenerators. For a halogen-doped ZnO nanowire film, dopants and dopingconcentration dependent lattice strain along the ZnO c-axis are establishedand confirmed by the EDS, XRD, and HRTEM analysis. Although latticestrain induced charge separation was theoretically proposed, it has not beenexperimentally investigated for wurtzite structured ZnO nanomaterials.Tuning the lattice strain from compressive to tensile state along the ZnO c-axis can be achieved by a substitution of halogen dopant from fluorine to otherhalogen elements due to the ionic size difference between dopants andoxygen. With its focus on a group of nonmetal element induced lattice strainin ZnO-based nanomaterials, this work paves the way for enhancing theperformance of wurtzite-type piezoelectric semiconductor nanomaterials vialattice strain strategy which can be employed to construct piezoelectric nanodevices with higher efficiency in a cost-effectivemanner.

KEYWORDS: piezoelectric nanogenerator, ZnO, lattice strain, piezocharges separation, chemical doping

1. INTRODUCTION

Recent advances in material science enable us to harvestambient mechanical energy from the environment, convert itinto electric energy, and then power portable electronics.1,2

Piezoelectric nanogenerators (PENGs) and triboelectric nano-generators (TENGs) have attracted a great deal of attentionfrom the scientific community and are considered as promisingbuilding blocks for the design and application of renewable,lightweight, and low-cost energy sources.3−5 The outputperformance of the nanogenerator is highly dependent onnanomaterials employed in TENGs or PENGs.5−9 With respectto large piezoelectric coefficients in PZT, PMN-PT, and PZN-PT materials, there is always a strong push to explore lead-freebuilding materials because of environmental concerns.10−15

Therefore, it is essential to develop high-performance PENGsand TENGs with environmental friendly materials, especiallyfor the case of implantable or human skin wearable energyharvesting systems.16−21

ZnO is one of the most fascinating environmentally benignfunctional materials with distinguished performance in PENGsand piezotronic devices, such as piezoelectric field effecttransistors, piezoelectric diodes, piezoelectric strain sensors,

biological sensors, and photodetectors.1,22−24 Therefore, theoutput performance of a ZnO-based piezoelectric nanodeviceshould be further optimized by modulating the characteristic ofintrinsic ZnO.25 In order to improve the utilization efficiency ofpiezoelectric charge, it is indispensable to synthesize non-centrosymmetric materials to broaden the investigation of thep- and n-type doped ZnO.26 For instance, Sb-doped ZnOnanowire/belt exhibited its potential applications in thermo-electric nanogenerators, self-powered ultraviolet photodetec-tors, and gesture recognition devices.27 Recent works revealedthat the optical and electronic properties of semiconductornanomaterials can be tuned via strain-engineering, e.g., buckledCdS nanowires, bent ZnO nanowires, and strain-driven redoxbehaviors in carbon nanomaterials.28−33 It is known thatpiezocharges can be generated by externally applied tensile orcompressive strain to the ZnO micro/nanowire or thin film, butless attention has been paid to the piezoelectric effect enhancedby lattice strain along the polar c-axis direction of wurtzite

Received: October 28, 2015Accepted: December 24, 2015Published: December 24, 2015

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© 2015 American Chemical Society 1381 DOI: 10.1021/acsami.5b10345ACS Appl. Mater. Interfaces 2016, 8, 1381−1387

ZnO.34 It has become more compelling that chemical doping isone of the prevalent approaches in tailoring the optical,electrical, and magnetic properties achieved by diverse dopantsand different doping concentration. Such advantages makechemical doping a promising route toward boosting theperformance of ZnO-based piezoelectric nanogenerators andpiezo-phototronic nanodevices. Considering the notable differ-ence in ionic radii between oxygen and halogen ions (F−, Cl−,Br−, and I−), it would be expected that the compressive ortensile lattice strain can be achieved in a doped ZnO film thathas an inevitable impact on the fundamental properties ofdoped ZnO materials and performance of ZnO-based piezo-electric nanodevice.In this work, we highlight the unique synthetic strategy which

leads to the enhancement of the output performance of ZnO-based flexible piezoelectric nanogenerator through tailoring thelattice strain along the ZnO polar c-axis. In a comparison to thestudy of F- and Cl-doped ZnO, the investigations of Br- and I-doped ZnO materials are rather scarce.35,36 More importantly, asystematical investigation on a group of halogen elements asnonmetal dopants has been difficult, which may open a newdimension in the field of ZnO-based piezoelectric nanomateri-als. Focusing on the lattice strain induced by different halogendopants, we characterized a series of halogen element dopedZnO nanowire films by energy dispersive X-ray (EDX)spectroscopy, X-ray diffraction (XRD), X-ray photoelectronspectroscopy (XPS), photoluminescence spectroscopy (PL),and high-resolution transmission electron microscopy(HRTEM). The electron transport property of the halogen-

doped ZnO nanowire film was investigated as well. Ourexperimental observation highlights the essential role of variedhalogen dopants as the key element in inducing the latticestrain along the ZnO c-axis. The current synthetic strategy is instark contrast to the previous reports with attentionconcentrated on metal-doped ZnO as the building block inPENG, but the role of dopant with different ionic size remainsunexplored.37,38

2. EXPERIMENTAL SECTION2.1. Material Synthesis and Characterization. The synthesis

process of undoped and halogen-doped nanowire films is carried outby a modified hydrothermal method. First, a ZnO seed layer wasdeposited by standard RF magnetron sputter deposition using a high-purity ZnO target (99.99%) and Ar/O2 (flow rate was fixed at 8:1) asthe sputtering gases at ambient temperature. Second, the PET withZnO seed layer was placed in the nutrient solution consisting of zincnitrate hexahydrate (100 mM) and hexamethylenetetramine (100mM) and 10 mM NaX (X = F, Cl, Br, and I). After 7 h reaction in anoven (at 95 °C), the as-prepared ZnO nanowire films were carefullyrinsed by deionized water to remove the unreacted chemical on thesurface of the sample. Prior to the assembly of flexible PENG, theundoped and halogen-doped ZnO nanowire films were placed in 40°C conditions overnight. The morphology of the halogen-doped ZnOnanowire was examined by TEM (F20). The crystal structure of theundoped and halogen-doped ZnO nanowire film was obtained byXRD using a Bede D1 ZM-SJ-001 diffractometer. The XPScharacterization was conducted by Thermo Scientific ESCALAB250Xi.

2.2. Device Fabrication and Characterization of PENG. Thefabrication of PENG was fulfilled by sputtering 100 nm Cu (99.99%

Figure 1. (a) TEM images of the ZnO:X nanowire, chemical mapping images of Zn, O, F, Cl, Br, and I. (b) EDX spectra of ZnO:X NW film (X = F,Cl, Br, and I).

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purity) on the top of ZnO nanowire film and on the bottom of PET.In order to protect the Cu electrode and PENG during measurement,the PDMS layers were deposited on the top and bottom sides of theflexible PENG. The photograph of a ZnO-based PENG was presentedin Figure S1 (see Supporting Information). The output voltage andcurrent of the PENG were characterized by a LeCroy 610Zioscilloscope, Stanford Research System SR 570, low-noise currentamplifier. The room temperature electrical property was measured by asemiconductor characterization system (Keithley 4200-SCS).

3. RESULTS AND DISCUSSION

All of the halogen-doped ZnO nanowire (NW) films weresynthesized by the modified hydrothermal routine in thepresence of NaX (X = F, Cl, Br, and I). A detailed discussion ofsynthesis and fabrication of doped ZnO NW film PENG isdescribed in the Experimental Section. Hereafter, thesynthesized materials will be abbreviated as ZnO:X, where Xdenotes the corresponding halogen element (e.g., ZnO:Br isdenoted as Br-doped ZnO). All as-synthesized doped ZnOcompounds were synthesized using the hydrothermal methodin the present of NaX containing nutrient solution. Figure 1shows transmission electron microscopy (TEM) images ofhalogen-doped ZnO nanowires that were peeled off from theZnO:X NW films. The chemical mapping images pointed outthe distribution of halogen dopant throughout the whole ZnOnanowire in each sample, Figure 1a. The TEM characterizationindicates the ZnO:X nanowire served as the building block inthe textured films. The surface features of the as-synthesizedZnO:X NW film are similar to the undoped ZnO NW film. TheEDX and XPS spectra of halogen-doped ZnO NW filmexhibited signals from halogen dopant in each sample whichagree well with TEM characterization. The doping concen-tration in each sample is 0.10% in ZnO:F, 0.20% in ZnO:Cl,0.15% in ZnO:Br, and 0.08% in ZnO:I, Figure 1b and Figure S2(SI).39 Due to the small atomic number of fluorine, EDXcharacterization only can give the qualitative result on thepresentation of F dopant, which is influenced by the samplesize, quality, and its low doping concentration.40,41 Figure S3

(SI) shows the PL spectra of undoped ZnO and ZnO:X NWfilm excited by a He−Cd laser at 325 nm. For ZnO:X NW film,the observation of the strong emission peak in the ultravioletand broad green emission peaks in the visible region wascomparable to previous reports.42 The considerable differencewas shown in the visible part as the incorporation of halogendopant due to its different concentration and ionic size.Recently, Wang and co-workers reported that a heavily dopedZnO single nanowire with chlorine ions exhibited metallicconductivity. Compared to that of the undoped ZnO film, theconductivity of ZnO:X slightly increases by selecting a differentdopant source in nutrient solution, see Figure S4 (SI). Themorphology of undoped ZnO and Br-doped ZnO NW film wascharacterized. The cross-sectional and top views of ZnO NWfilm are presented in Figure 2 and Figures S5 and S6,respectively. The above-mentioned results provide solidevidence that the halogen ions were doped into the ZnOlattice in the process of hydrothermal synthesis and the dopingconcentration of halogen ions is quite low in ZnO:X.The output performance of PENG essentially depends on

the sandwiched piezoelectric materials between two electrodes.The device structure is schematically presented in Figure 2c, aswell as the comparison of their output performance. The PETlayer is multifunctional in the design of PENG including servingas flexible platform and avoiding the electron leakage. Withapplication of a stress (1 MPa) to the PENG surface, thepositive and negative piezocharges are generated andaccumulated at the ZnO:X/PET interface and ZnO:X/Cuinterface, respectively. The obtained Vpp (peak-to-peak voltage)and Ipp (peak-to-peak current) values for as-grown ZnO areestimated at 2.40 V and 400 nA/cm2, respectively. Thepiezoelectric output performance of ZnO:X NW film isdemonstrated in comparison to that of undoped ZnO NWfilm as well. When NaBr was added into the nutrient solution,the performance of the ZnO:Br NW film can be achieved in theaspects of both output voltage (up to 5.90 V) and current (upto 1910 nA/cm2). The measurements of the output of ZnO

Figure 2. SEM cross-sectional view of (a) undoped ZnO and (b) ZnO:Br NW film. The scale bar is 5 μm. (c) Schematic diagram of the structure ofhalogen-doped ZnO NW film PENG. (d) Output voltage and (e) current of undoped ZnO and halogen-doped ZnO NW film as flexiblepiezoelectric nanogenerators.

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NW film PENG with and without halogen element dopantswere conducted under repeatedly applying a stress more than30 cycles, see Figure S7 (SI). When the same amount of NaFwas employed as the doping source, the output performance ofZnO:F NW film is lower than that of undoped ZnO NW filmand other ZnO:X (X = Cl, Br, and I) NW films, see Figure 2d,e.The influence of different concentrations of the same halogendopant on the piezoelectric performance improvement wascarried out as well. The chlorine was chosen as the dopant toprepare ZnO:Cl NW films. With an increase in the NaClconcentration in nutrient solution, the enhancement in theoutput voltage of ZnO:Cl NW film was obtained, see Figure S8.According to the previous reports, the morphology, density,and size of ZnO NW will have impact on the outputperformance of PENG.43 The length of the ZnO NW or thethickness of the ZnO NW film will influence the output voltage,while the diameters of ZnO NW will influence the outputcurrent. On the basis of our results, all of as-prepared undopedZnO and halogen-doped ZnO films are composed of closelycompacted ZnO nanowires with identical thickness anddiameters mainly ranging from 150 to 350 nm, see Figure2a,b. The output performance of undoped and doped ZnO NWtextured film depends on the film thickness, which is differentfrom the case of ZnO NW arrays film. Because we synthesized ahalogen-doped ZnO NW film with identical thickness and fullycovered on the PET substrate (see Figure S6), it can beconsidered that the difference in their piezoelectric perform-ance resulted from the corresponding dopant.The crystal structure of ZnO NW film was analyzed by X-ray

diffraction using Cu Kα radiation (λ = 1.5406 Å). In order toaccurately evaluate the influence of halogen dopant on thecrystal nature of ZnO NW film, the diffraction peaks had beencalibrated by the peak position of Si (004) for the as-preparedsamples, see Figure 3. The lattice spacings of ZnO (0002)calculated from the diffraction patterns are 5.2058 Å in

undoped sample while 5.2040 Å in ZnO:F, 5.2214 Å inZnO:Cl, 5.2291 Å ZnO:Br, and 5.2139 Å in ZnO:I. Figure 3c,dshows HRTEM images of the individual undoped and ZnO:Brnanowire which were peeled from a NW film. In a comparisonto the undoped ZnO sample, the slight change of c parameterscan be attributed to the incorporation of halogen ions in ZnONW film in the process of the hydrothermal synthesis. In fact, itwould be expected that the corresponding ZnO (0002) planespacing will be tuned due to the difference in ionic radiusbetween oxygen ions (r(O2−) = 0.140 nm) and halogen ions(r(F−) = 0.133 nm, r(Cl−) = 0.181 nm, r(Br−) = 0.193 nm, andr(I−) = 0.220 nm)) when the oxygen ions are replaced byhalogen ions. We notice the lattice strain along ZnO polar c-axishas two contributions which can be interpreted by simulta-neous analysis of the ionic size of dopant and doingconcentration.In comparison to vapor deposition techniques, hydrothermal

synthesis has its advantage in the preparation of doped ZnOnanomaterials in low temperature.35,44 As revealed from recentwork, the doping concentration of chlorine ions can becontrolled around 0.20% regardless of catalytic effect fromother cations or anions. In nutrient solution, halogen ions X−

act as the dopant source, diffuse and bind to the surface of ZnONW film, and then form Zn(OH)xXy

(x+y−2) from Zn(OH)2+xx−

species temporarily. After the deposition of Zn(OH)2+xx−/

Zn(OH)xXy(x+y−2)− and dehydration of the OH− group,

halogen ions were permanently preserved in the ZnO lattice,see Figure 3e. We suspect their different doping concentrationresulted from the ionic radii of the varied halogen dopants thatinfluence the amount of intermediate Zn(OH)xXy

(x+y−2)−

forming in the synthetic process.

= + + +⎛⎝⎜

⎞⎠⎟d

l hk ka

lc

1 43hkl( )

2

2 2

2

2

2(1)

Figure 3. (a, b) XRD spectra of ZnO and halogen-doped ZnO NW film; (c, d) HRTEM images of the individual undoped ZnO and ZnO:Brnanowire which peeled from NW film, the scale bar is 5 nm; (e) crystal structure of ZnO:X (X = F, Cl, Br, and I).

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λθ

=csin (2)

=−

×cc c

c100%zz

0

0 (3)

Under our conditions, the strain along the c-axis, czz, is givenby eqs 1−3, where c is the lattice parameter of ZnO:X NWfilms calculated from X-ray diffraction data and c0 is the latticeparameter of undoped ZnO synthesized by the same methodwithout dopant source. The value of czz is used as a sensitiveparameter to understand the variation of the strain state in theZnO lattice.45 In this way, the calculated strain in halogen-doped ZnO is about −0.035% in ZnO:F, 0.30% in ZnO:Cl,0.45% in ZnO:Br, and 0.16% in ZnO:I. Even for iodine, withlarger ionic radius than chlorine and bromine, the low dopingconcentration results in the relatively smaller lattice strain inZnO:I NW film in comparison to ZnO:Cl and ZnO:Br samples,see Figure 4.

According to previous work on the individual ZnO nanowire,the local lattice spacing can be tuned by the elastic bendingdeformation, and the maximum strain was achieved about1.0%.34,46,47 The calculated lattice strain in ZnO:X samples islower than that in the case of an externally bending ZnOnanowire which can be explained as the presentation of thelimited amount of halogen ions along the c-axis of the ZnOnanowire. It is found that the compressive strain can beobtained by choosing F as the dopant since the ionic radius ofF− is slightly smaller than that of O2−. These results are in goodagreement with the ionic radius difference because the ionicradius of O2− is larger than that of F−, but smaller than those ofCl−, Br−, and I−. With the undoped ZnO sample taken intoaccount, the lattice strain in the ZnO:X varies fromcompression to tension state which was modulated bysubstituting the selected dopant from F to Cl, Br, and I ionswith apparently increasing their ionic radii. Halogen ions playan indispensable role in modulating lattice strain along the ZnOpolar c-axis. Previous works described the scenario of utilizingstrain for charge separation in semiconductor materials at thenanoscale.48−50 The ZnO:X nanomaterials synthesized in ourcondition have demonstrated that both ionic size and dopingconcentration were found as the driving force for increasingand decreasing the lattice strain along ZnO polar axis.Therefore, it can be concluded that the maximum lattice strain

depends on the selected dopant as well as its dopingconcentration. With ionic radius larger than oxygen’s, theexistence of halogen element in ZnO lattice leads to the tensilestrain that naturally causes a static polarization to the ZnOlattice and thus the piezocharges at the two ends. However, itshould be mentioned that the dopant can induce the latticedistortion and the lattice disorder as well, which may have anegative contribution on the performance of ZnO-basedPENG.43,51 Precise estimation of the lattice strain and disordercontribution to ZnO:X film requires an in situ study on ZnO:Xnanowires, which is underway, and complete analysis of thesefactors will be presented in the future. According to previousstudies, the piezoelectric potential screening by free electrons inundoped ZnO has limited the energy conversion efficiency ofthe ZnO-based PENGs.52−55 The deposition of CuO or otherp-type materials can consume the free electrons in ZnO layerand further improve the output performance of PENG. Due tothe CuO/ZnO pn heterojunction, a depletion region is formednear the junction boundary. Therefore, the built-in electric fieldnear the heterojunction/interface effectively depletes freeelectrons in the ZnO layer, which reduces the screening effectsfor the generated piezoelectric potential and leads to anenhancement of piezoelectric output voltage. In order toillustrate whether chemical doping increases the electronscreening effect, we synthesized undoped ZnO, ZnO:Cl, andZnO:Br nanowire films on a predeposited CuO layer. Theenhanced output performances were obtained for ZnOnanowires film with and without halogen dopants. For undopedZnO NW film, the output voltage increased from 2.4 to 4.8 V.For ZnO:Cl and ZnO:Br NW film, their output voltagesincreased from 4.9 to 7.2 V and from 5.9 to 8.1 V, respectively.The enhancement of halogen-doped ZnO NW film iscomparable to the undoped ZnO NW film, which indicatesthe chemical doping does not cause a significant electronscreening effect and the improvement in their piezoelectricoriginated from the corresponding dopants. The larger XRDshift and lattice strain can be achieved by increasing the ionicsize of dopants and doping concentration. However, if the ionicsize of the dopant is too large and/or the doping concentrationis too high, the existence of dopants may result in more latticedefects, which increases the concentration of free electrons andthe screening effect and decreases the output performance. Inour case, the halogen element doped ZnO NW film can havelarger lattice strain and without extra electron screening effect.Therefore, dopants (Cl, Br, and I) played an essential role inenhancing the piezoelectric output of ZnO NW film. Forinstance, previous work demonstrated that the S-doped ZnONW demonstrated a large XRD shift and high dopingconcentration, which exhibited slight enhancement in piezo-electric output compared to that of the undoped ZnO NW.43

This indicates chemical doping can be regarded as a practicalroutine for enhancing piezoelectric performance only bycontrolling the ionic size of dopants and controlling the dopingconcentration. The dopant induced the lattice strain, and theelectron screening effect has a different contribution to themechanically generated piezopotential, which is a competitiveprocess. For the enhancement of the performance ofpiezoelectric nanogenerator and nanodevice, introducing p-type nanomaterials onto the ZnO film has been reportedrecently.47−49 However, our synthetic strategy achieved theobjective of enhancing the piezoelectric performance of ZnOfilm in a one-step solution process at low cost.54−56

Figure 4. ZnO lattice strain dependence of the output performance ofpiezoelectric nanogenerator.

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4. CONCLUSIONS

We have performed a unique synthetic strategy that modulatesthe lattice strain along ZnO polar c-axis by a low-cost chemicaldoping approach. Tuning the lattice strain from a compressiveto tensile state can be achieved by the variation of halogendopant from fluorine to other halogen elements by takingadvantage of the ionic size between the dopant and oxygenelement. For halogen-doped ZnO nanowire film, dopants anddoping concentration-related lattice strain along ZnO c-axis areestablished and confirmed by the experimental characterization.As the successful and comprehensive investigation on a groupof nonmetal element induced lattice strain in ZnO-basedfunctional materials, this work paves the way for enhancing theperformance of wurtzite-type piezoelectric semiconductornanomaterials via lattice strain which can be potentiallyemployed to construct piezoelectric nanodevices with higherefficiency in a cost-effective manner.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.5b10345.

XPS, PL spectra, I−V curves, output performance ofPENGs, and SEM images of doped ZnO NW films(PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: jyzhai@binn.cas.cn.*E-mail: zlwang@gatech.edu.

Author ContributionsYang Zhang and Caihong Liu contributed equally.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by NSFC (51472056, 51402064), the“Thousands Talents” program for pioneer researcher and hisinnovation team, China, and the Recruitment Program ofGlobal Youth Experts, China.

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ACS Applied Materials & Interfaces Research Article

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